How severe is CVE-2026-3590?

CVE-2026-3590 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-3590?

CVE-2026-3590 affects the following packages: github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-3590?

Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20250723052842-4cb8d8940332 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-3590 is resolved across your whole dependency graph.

How do I detect CVE-2026-3590 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-3590 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-3590?

O3 pinpoints whether CVE-2026-3590 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-3590 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-3590 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-3590 published, and has it been updated?

CVE-2026-3590 was published on April 15, 2026 and was last updated on July 31, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

CVE-2026-3590

MEDIUM

CVE-2026-3590 is a medium-severity (CVSS 6.5) vulnerability in github.com/mattermost/mattermost/server/v8. O3 Security confirms whether CVE-2026-3590 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Race Condition in Guest Magic Link Authentication Allows Token Reuse

Also known asGHSA-mh4x-rmrx-3hp4GO-2026-5507

Published

Apr 15, 2026

Updated

Jul 31, 2026

Affected

5 pkgs

Patched

5 / 5

Exploits

None indexed

Blast Radius

5 pkgs affected

🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624

Affected Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 8.0.0-20250721062209-4952acea88ce&&< 8.0.0-20250723052842-4cb8d8940332	8.0.0-20250723052842-4cb8d8940332
🐹Go	`github.com/mattermost/mattermost-server`	≥ 10.11.0-rc1&&< 10.11.13	10.11.13
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.5.0-rc1&&< 11.5.0	11.5.0
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.4.0-rc1&&< 11.4.3	11.4.3
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.3.0-rc1&&< 11.3.3	11.3.3

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20250723052842-4cb8d8940332 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-3590 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-3590 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-3590. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-3590 in your dependencies?

O3 detects CVE-2026-3590 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works